What are track codes and which is best – OO Gauge Code 100 or 75

Along with gauge, track codes are one of the most misunderstood concepts for beginners in model railways. Andy Carter from Calling All Stations decodes the codes.

Note: This applies to OO gauge model railway track. A note on N gauge is at the bottom of this article.

OO Gauge suffers from a historical anomaly that means it’s rolling stock is actually too big for the track it runs on. In the early 20th-century British model manufacturers had decided to compete with their American counterparts who had just launched O gauge. (Half of O Gauge, 3.5mm to the foot scale or 1:87).

However, to keep costs down they were still manufacturing models with wind-up mechanisms.

This method of propulsion was proving hard to fit into 1:87 scale models.

The solution was to enlarge the models to 1:76 scale (or 4mm to the foot). Instead of enlarging the track with it – a costly exercise, the British manufacturers simply continued to import the American HO track and built the models’ axle width to cope accordingly.

At the time, when manufacturing techniques were cruder than they are today, this inaccuracy was not so noticeable.

Unfortunately, the mismatch in scale sizes stuck and is still in use by all the major companies (Hornby, Bachmann, Dapol, Heljan etc) today. The inaccuracy may still be hard to spot with the majority of models, but put a British OO Gauge model next to an American or European HO counterpart on the same track and the difference will certainly be noticeable.

So what can be done?

Some modellers do what the British manufacturers never did and properly enlarge the HO track to its true 1:76 scale. This is known as EM Gauge (and an even more accurate scale is P4), although you might often hear it referred to as ‘Fine Scale.’

This is a rather drastic step and almost exclusively requires the scratch built track to be constructed by the modeller in question. It also requires all the wheelbases and axle widths to be altered on the models themselves to cope with the enlarged track. The results are certainly remarkable, but this is usually beyond the Novice modeller such as myself.

Track Codes: What’s the difference?

An alternative is to look at improving the rail dimensions without altering the actual gauge.

You may have heard people talking about Track Codes – these are alternative varieties of off-the-shelf HO/OO Track with different rail heights.

The gauge is exactly the same, but the rail height varies in an attempt to be more accurate.

Code 100

The ‘100’ simply means that the rail is 0.1 inches high. If this were to be scaled up alongside OO Gauge rolling stock, the rail height would be almost 30% higher than its real-life equivalent.

If you scaled this up it would this time be 3% lower than the real thing. It’s still not perfect, but it’s much closer to the mark than Set Track.

Codes 100 and 75 are the most common but there are other varieties:

Code 83 is used to improve the accuracy of North-American-Image rail heights, while

Code 60 can be used on UK-Image layouts to mimic 3rd and 4th rail systems.

The track you get in any Hornby, Bachmann or Peco starter set is almost definitely Code 100 (sometimes referred to as ‘Set Track’).

Do you need to worry about the difference?

Absolutely not.

Many great show grade models have used Code 100 track.

When ballasted properly you can still achieve a great effect, and it certainly looked good on Langstead Junction… However, Code 75 track will look that tiny bit better still. To help you make up your own mind I’ve photographed some Code 75track alongside Code 100 track.

In the first picture, the shallower rail height is certainly noticeable on the Code 75 version. The sleepers also look less chunky and better spaced.

Code 100 Top, Code 75 Bottom

The difference in rail height is really noticeable side on:

Code 100, Left. Code 75, Right.

With trains added, the Code 100 doesn’t look too bad and from this angle, the difference is less noticeable:

Code 100, Left. Code 75, Right.

And from above.

Code 100, Left. Code 75, right

From some angles Code 75 looks noticeably better, but for others the difference is hard to tell.

Along with looks, there are other differences.

Code 100 is tougher, cheaper and curved radius’s come in pre-formed pieces (or Set Track to you and me), whereas Code 75 is only available in long strips of Flexi-track and can, therefore, be more challenging to create authentic looking curved sections.

In summary, as with most things in modelling, there are pros and cons to each side of the argument.

I’ve decided to use Code 75 from Peco for my new project, Woodford Wells, as the majority of the track is fairly straight and easy to shape.

If you want to read more about Track Codes I strongly recommend visiting Model Rail Workshop to read the full study and comparison. This post certainly helped me!

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Good explanation, but nothing said about wheels and flange clearances, (as comparisons for example), between Code 100 and Code 75. Not particularly critical I would have thought on straight track, but maybe a problem on some curves and at point work……..or am I mistaken and even on straights there might be a problem with wheel flanges striking or continually rubbing on sleeper chairs.

At some exhibitions and on perhaps a few layouts I often see rolling stock wobbling a bit too much……….and only running at slow speeds, (and yes, I do recognise here that slower scale speeds might be more appropriate and correct in operational terms), but is this something that has to be done to avoid derailments…?

I suppose I am asking here if faster running is only possible with higher rail levels and the appropriately profiled wheels.

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I’m Andy, welcome to my multi-award winning website for model making I share the best tips, inspiring photos, comprehensive guides and stories from my adventures in making miniature worlds, miniatures and model railways. Read more about me, model making tips or learn how to start your own model railway.